1. Field of the Invention
The present invention relates to a technique of annealing, for instance, a semiconductor material uniformly and efficiently over a large area. The invention also relates to a technique of preventing reduction of processing efficiency in illuminating a particular region while gradually changing the illumination energy density.
2. Description of the Related Art
In recent years, extensive studies have been made of the temperature reduction of semiconductor device manufacturing processes. This is largely due to the need of forming semiconductor devices on an insulative substrate, such as a glass substrate, which is inexpensive and superior in workability. Other needs such as needs of forming finer devices and multilayered devices have also prompted the studies on the process temperature reduction.
In particular, a technique of forming semiconductor devices on a glass substrate is necessary to produce a panel that constitutes an active matrix liquid crystal display device. This is a configuration in which thin-film transistors are formed on a glass substrate so as to assume a matrix of more than several hundred by several hundred. When a glass is exposed to an atmosphere of more than about 600° C., deformation such as contraction and strain becomes remarkable. Therefore, the heating temperature in a thin-film transistor manufacturing process should be as low as possible.
To obtain thin-film transistors having superior electrical characteristics, a crystalline thin-film semiconductor needs to be used.
Among methods of producing a crystalline silicon film is a technique of crystallizing, by a heat treatment, an amorphous silicon film that has been deposited by plasma CVD or low-pressure thermal CVD of about 500° C. This heat treatment is such that a sample is left in an atmosphere of 600° C. or more for more than several hours. In this heat treatment, where the temperature is, for instance, 600° C., long process time of more than 10 hours is needed. In general, if a glass substrate is heated at 600° C. for more than 10 hours, deformation (strain and contraction) of the substrate becomes remarkable. Since a thin-film semiconductor for constituting thin-film transistors is several hundred angstrom in thickness and several micrometers to several tens of micrometers in size, the substrate deformation will cause an operation failure, a variation in electrical characteristics, or the like. In particular, in the case of a large-sized substrate (diagonal size: 20 inches or more), the substrate deformation is a serious problem.
If the heat treatment temperature is higher than 1,000° C., crystallization can be attained in a process time of several hours. However, ordinary glass substrates cannot withstand a high temperature of about 1,000° C. even if a heat treatment lasts for a short time.
Quartz substrates can withstand a heat treatment of more than 1,000° C., and allow production of a silicon film having superior crystallinity. However, large-area quartz substrates are particularly expensive. Therefore, from the economical point of view, they cannot be easily applied to liquid crystal display devices, which will be required to be increased in size in the future.
In the above circumstances, the temperature of processes for manufacturing thin-film transistors is now required to be lowered. Among techniques for attaining this purpose is an annealing technique that uses laser light illumination, which technique now attracts much attention with a possibility of providing an ultimate low-temperature process. Since laser light can impart as high energy as thermal annealing to only a necessary portion, it is not necessary to expose the entire substrate to a high-temperature atmosphere. Therefore, the annealing technique by laser light illumination enables use of glass substrates.
However, the annealing technique by laser light illumination has a problem of unstable laser light illumination energy. Although this problem can be solved by using a laser apparatus capable of emitting laser light of higher energy than necessary one and attenuating the output laser light, there remains another problem of cost increase due to increased size of the laser apparatus.
Even with such a problem, the annealing technique by laser light illumination is still very advantageous in that it enables use of glass substrates.
In general, there are two laser light illumination methods described below.
In a first method, a CW laser such as an argon ion laser is used and a spot-like beam is applied to a semiconductor material. A semiconductor material is crystallized such that it is melted and then solidified gradually due to a sloped energy profile of a beam and its movement.
In a second method, a pulsed oscillation laser such as an excimer laser is used. A semiconductor material is crystallized such that it is instantaneously melted by application of a high-energy laser pulse and then solidified.
The first method of using a CW laser has a problem of long processing time, because the maximum energy of the CW laser is insufficient and therefore the beam spot size is at most several millimeters by several millimeters. In contrast, the second method using a pulsed oscillation laser can provide high mass-productivity, because the maximum energy of the laser is very high and therefore the beam spot size can be made several square centimeters or larger.
However, in the second method, to process a single, large-area substrate with an ordinary square or rectangular beam, the beam needs to be moved in the four orthogonal directions, which inconvenience still remains to be solved from the viewpoint of mass-productivity.
This aspect can be greatly improved by deforming a laser beam into a linear shape that is longer than the width of a subject substrate, and scanning the substrate with such a deformed beam.
The remaining problem is insufficient uniformity of laser light illumination effects. The following measures are taken to improve the uniformity. A first measure is to make the beam profile as close to a rectangular one as possible by causing a laser beam to pass through a slit, to thereby reduce an intensity variation within a linear beam. A second measure to further improve the uniformity is to perform preliminary illumination with pulse laser light that is weaker than that of subsequently performed main illumination. This measure is so effective that the characteristics of resulting semiconductor devices can be improved very much.
The reason why the above two-step illumination is effective is that a semiconductor material film including many amorphous portions has a laser energy absorption ratio that is much different than a polycrystalline film. For example, a common amorphous silicon film (a-Si film) contains hydrogen at 20 to 30 atomic percent. If laser light having high energy is abruptly applied to an amorphous silicon film, hydrogen is ejected therefrom, so that the surface of the film is roughened, i.e., formed with asperities of several tens of angstrom to several hundred angstrom. Since a thin-film semiconductor for a thin-film transistor is several hundred angstrom in thickness, its surface having asperities of several tens of angstrom to several hundred angstrom will be a major cause of variations in electrical characteristics etc.
Where the two-step illumination is performed, a process proceeds such that a certain part of hydrogen is removed from an amorphous silicon film by the weak preliminary illumination and crystallization is effected by the main illumination. Since the illumination energy is not high in the preliminary illumination, there does not occur severe surface roughening of the film due to sudden hydrogen ejection.
The uniformity of the laser light illumination effects can be improved considerably. However, if the above two-step illumination is employed, the laser processing time is doubled, thus reducing throughput. Further, since a pulsed laser is used, some variation occurs in the laser annealing effects depending on the registration accuracy of the main illumination and preliminary illumination, which variation may greatly influence the characteristics of thin-film transistors having a size of several tens of micrometers by several tens of micrometers.
In general, among various processing techniques (for example, causing a quality change in various materials and processing by application of laser energy) by laser light illumination is a technique in which a certain region is illuminated plural times with laser beams of varied energies. The above-described annealing technique for a silicon film is an example of such a technique.
Conventionally, in such a technique, a laser beam is applied plural times, which however elongates the processing time by a factor of the number of illumination times, and causes a large decrease in the operation efficiency. Further, illuminating a particular region plural times with laser beams likely causes a problem of a deviation of illumination areas, and is not practical because solving this problem may be technically difficult or may require a costly technique.